1. Field of the Invention
The present invention relates to a method of semiconductor manufacturing in which doping is accomplished by the implantation of ion beams formed from ionized molecules, and more particularly to a method in which molecular and cluster dopant ions are implanted into a substrate with and without a co-implant of non-dopant cluster ion, such as a carbon cluster ion, wherein the dopant ion is implanted into the amorphous layer created by the co-implant in order to reduce defects in the crystalline structure, thus reducing the leakage current and improving performance of the semiconductor junctions, the dopant ions being of the form AnHx+, or AnRzHx+, where n, x and z are integers with n greater than or equal to 4 and x and z greater than or equal to 0, and A is carbon, boron, indium, arsenic, phosphorus, or antimony and R is a non-dopant molecule, radical or ligand comprised of atoms such as Si, Ge, F, H or C which are not injurious to the implantation and doping process, or to device performance. The present invention utilizes the auto-amorphization properties of clusters in order to ameliorate the problem of annealing out the implant damage caused by prior art Ge pre-amorphization implants. These species, either alone or in combination, allow for the formation of defect-free USJ's of both n- and p-type.
2. Description of the Prior Art
The Ion Implantation Process
The fabrication of semiconductor devices involves, in part, the introduction of specified impurities into the semiconductor substrate to form doped regions. The impurity elements are selected to bond appropriately with the semiconductor material so as to create electrical carriers. This introduction alters the electrical conductivity of the semiconductor material in the “doped” region. The concentration of dopant impurities so introduced determines the electrical conductivity of the resultant region. The electrical carriers can either be electrons (generated by N-type dopants) or holes (generated by P-type dopants). Many such N- and P-type impurity regions must be created to form transistor structures, isolation structures and other such electronic structures, which function collectively as a semiconductor device.
Ion implantation is the conventional method of introducing dopants into a semiconductor substrate. In ion implantation, a feed material containing the desired element is introduced into an ion source and energy is supplied to ionize the feed material, creating ions which contain the dopant element. For example, in silicon, the elements As, P, and Sb are donors or N-type dopants, while B and In are acceptors or P-type dopants.
An accelerating electric field is provided to extract and accelerate the ions, thus creating an ion beam. Typically, the ions contain a positive charge. However, in certain cases, negatively-charged ions may be used. Mass analysis is used to select the exact species to be implanted. The mass-analyzed ion beam may subsequently pass through ion optics which alter its final velocity or change its spatial distribution prior to being directed into a semiconductor substrate or work piece. The accelerated ions possess a well-defined kinetic energy which allows the ions to penetrate the target to a predetermined depth. Both the energy and mass of the ions determine their depth of penetration into the target. Higher energy and/or lower mass ions allow deeper penetration into the target due to their greater velocity.
The ion implantation system is constructed to carefully control the critical variables in the implantation process. Critical variables include: the ion acceleration, ion mass, ion beam current (electrical charge per unit time), and ion dose at the target (total number of ions per unit area that penetrate into the target). Beam angular divergence (the variation in the angles at which the ions strike the substrate) and beam spatial uniformity and extent must also be controlled in order to preserve semiconductor device yields.
Ion implantation is always followed by thermal heating or annealing step. The purpose of this step is two-fold. One, to activate the dopant that has been implanted into the semiconductor. Activation is the process of replacing a Si atom by a dopant atom in the crystal. This step is required to change the conductivity of the material. Two, correct damage to the crystal caused by the ion implantation process.
Damage to the crystal is caused by two energy loss mechanisms that reduce the energy (velocity) of the ions. Firstly there is electron energy loss where energy form the ion is transferred to the electrons in the material. This can cause point defects in the crystal. These defects are easily healed by thermal treatment of a few hundred degrees C. Secondly there is nuclear energy loss that occurs when the ion has a collision with a lattice atom. This results in momentum transfer to the lattice atom and can actually knock it out of place and give it a velocity which interns strikes another atom causing it to move. This cascade of displaced atoms can lead to extended defects. These defects are much more difficult to heal and require higher temperature treatments.
The probability of electronic energy transfer is much higher at high energy that at low energy and conversely the probability of a nuclear collision event is lower at high energy and higher at low energy. Therefore near the surface where the energy of the ion beam is higher, most of the defects are point defects, but deeper into the material where the energy has been reduced the defects are primarily due to nuclear collisions and are more difficult to anneal out. The defects at the end of the ion path are call end-of-range defects and are particularly difficult to anneal. Temperatures almost to the melting point are required to remove these defects. Such high temperature anneals are known to be detrimental to device structures because they cause long diffusion lengths which result in deeper dopant profiles than desired.
This dilemma of how to eliminate damage that diminishes device performance while at the same time not causing the dopants to diffuse deep into the silicon substrate has long been studied. Many annealing schemes have been developed since the invention of solid state devices. These schemes include low temperature long time anneals to very high temperature very short duration anneals such as sub-melting laser annealing, or liquid phase annealing, such as with flash lamps and lasers where the very near surface is melted. There have even been attempts to use microwaves and shock waves to anneal silicon that has been implanted. The goal is always high activation, shallow distributions of the dopant, and removal of residual crystalline damage.
The post anneal residual damage must be engineered to assure proper device performance. There is a strong interaction between the implant conditions, anneal conditions and ambient conditions during the anneal. During the implant, the species, the energy, the dose, the dose rate, the temperature, the orientation of the wafer with respect to the ion beam, and the angular uniformity of the ion beam all have an impact on the damage profile in the silicon crystal. The anneal temperature, ramp rate, the times and temperatures of thermal plateaus, the ramp rates between plateaus, the ramp rate between a plateau and the maximum temperature, the maximum temperature, the time at maximum temperature, and the quench rate all have effects on the damage structures and profiles. The chemical ambient during the anneal, as well as wavelength of the annealing energy, all affects the final state of the damage. This study and control of these variables, and their interaction with one another, is known as defect engineering. Its goal is to process the material in such as way as to use the damage for a positive result, as in gettering, or to minimize the residual damage as in the junction of a transistor where residual damage can lead to extraneous electrical paths causing leakage or crosstalk between adjacent transistors.
A key process of semiconductor manufacturing is the creation of P—N junctions within the semiconductor substrate. This requires the formation of adjacent regions of P-type and N-type doping. An important example of the formation of such a junction is the implantation of P or N-type dopants into a semiconductor region already containing a uniform distribution of one dopant type. In these cases, an important parameter is the junction depth. The junction depth is defined as: the depth from the semiconductor surface at which the P-type and N-type dopants have equal concentrations. This junction depth is a function of the implanted dopant mass, energy and dose.
An important aspect of modern semiconductor technology is the continuous evolution to smaller and faster devices. This process is called scaling. Scaling is driven by continuous advances in lithographic process methods, allowing the definition of smaller and smaller features in the semiconductor substrate which contains the integrated circuits. A generally accepted scaling theory has been developed to guide chip manufacturers in simultaneously resizing all design aspects of the semiconductor device: i.e., at each technology or scaling node. The greatest scaling impact on ion implantation processes is the scaling of junction depths. This requires decreasing the junction depth as the device dimensions are decreased, requiring shallower junctions as integrated circuit technology scales. This translates into the following requirement: ion implantation energies must be reduced with each scaling step. The extremely shallow junctions called for by modern, sub-100 nanometer (nm) devices are termed “Ultra-Shallow Junctions”, or USJ
Physical Limitations on Low-Energy Beam Transport
Due to the aggressive scaling of junction depths in CMOS processing, the ion energy required for many critical implants has decreased to the point that conventional ion implantation systems cannot maintain the desired wafer throughput. The limitations of conventional ion implantation systems at low beam energy are most evident in the extraction of ions from the ion source, and their subsequent transport through the implanter's beam line. Ion extraction is governed by the Child-Langmuir relation, which states that the extracted beam current density is proportional to the extraction voltage (i.e., beam energy at extraction) raised to the 3/2 power. Similar constraints affect the transport of the low-energy beam after extraction. A lower energy ion beam travels with a smaller velocity, hence for a given value of beam current the ions are closer together, i.e., the ion density increases. This can be seen from the relation J=ηev, where J is the ion beam current density in mA/cm2, η is the ion density in ions/cm−3, e is the electronic charge (=6.02×10−19 Coulombs), and v is the average ion velocity in cm/s. In addition, since the electrostatic forces between ions are inversely proportional to the square of the distance between them, electrostatic repulsion is much stronger at low energy, resulting in increased dispersion of the ion beam. This phenomenon is called “beam blow-up” and is the principal cause of beam loss in low-energy transport. Low-energy electrons present in the implanter beam line tend to be trapped by the positively-charged ion beam, compensating for space-charge blow-up during transport. Blow-up nevertheless still occurs, and is most pronounced in the presence of electrostatic focusing lenses, which tend to strip the loosely-bound, highly mobile compensating electrons from the beam. In particular, severe extraction and transport difficulties exist for light ions, such as the N-type dopants phosphorus and arsenic. Being lighter than arsenic, phosphorus atoms penetrate further into the substrate than many other atoms, including arsenic. Hence the required implantation energies for phosphorus are lower than for arsenic. In fact, extremely low implantation energies, as low as 1 keV, are being required for certain leading edge USJ processes.
Heavier species, specifically cluster molecules, not only provide increased beam currents, but in many cases tend to auto-amorphize the crystalline silicon lattice. This type of auto-amorphization has been shown to be beneficial to the activation of P-type dopants, such as boron, and should provide similar benefits for N-type dopants. Also, auto-amorphization reduces ion channeling, enabling a shallower junction than possible in crystalline silicon. In fact, the process of record for many USJ logic manufacturers consists of a pre-amorphization implant of Ge or Si prior to performing the conductive doping implants in order to obviate channeling effects. The use of Ge or Si pre-amorphization implants has been shown to create end-of-range defects which result in increased leakage currents in the fabricated devices. Large cluster or molecular implantation has recently shown promise in the reduction or elimination of the end-of-range damage form doping implants. The kinds of defects and their locations in the crystal can be controlled by modifying the cluster or molecule size and composition used in doping the crystal. Carbon containing molecular ions may also be used to pre-amorphize semiconductor substrates in a similar way that Si, Ge and doping clusters do, for example, as disclosed in commonly owned U.S. patent application Ser. No. 11/634,565, filed on Dec. 6, 2006, entitled “System and Method for the Manufacture of Semiconductor Devices by the Implantation of Carbon Clusters” by Wade A. Krull and Thomas N. Horsky. Additionally, the carbon is known to inhibit the diffusion of boron during the annealing process.
Molecular Ion Implantation
A technique to overcome the limitations imposed by the Child-Langmuir relation discussed above is to increase the transport energy is by ionizing a molecule containing the dopant of interest, rather than a single dopant atom. Upon entering the substrate, the molecule breaks up into its constituent atoms, sharing the energy of the molecule among the individual atoms according to their distribution in mass, while the kinetic energy of the molecule is higher during transport, the dopant atom's implantation energy is much lower than the original transport kinetic energy of the molecular ion. Consider the dopant atom “X” bound to a radical “Y” (disregarding for purposes of discussion the issue of whether “Y” affects the device-forming process). If the ion XY+ were implanted in lieu of X+, then XY+ must be extracted and transported at a higher energy. The increase is by a factor equal to the mass of XY divided by the mass of X. This ensures that the velocity of X in either case is the same. Since the space-charge effects described by the Child-Langmuir relation discussed above are super-linear with respect to ion energy, the maximum transportable ion current is increased. Historically, the use of polyatomic molecules to ameliorate the problems of low energy implantation is well known in the art. A common example has been the use of the BF2+ molecular ion for the implantation of low-energy boron, in lieu of B+. This process dissociates BF3 feed gas to the BF2+ ion for implantation. In this way, the ion mass is increased to 49 AMU from 11 AMU. This increases the extraction and transport energy by more than a factor of 4 (i.e., 49/11) over using single boron atoms. Upon implantation, however, the boron energy is reduced by the same factor of (49/11). It is worthy of note that this approach does not reduce the current density in the beam, since there is only one boron atom per unit charge in the beam. A detriment to this process is the implanting of fluorine atoms into the semiconductor substrate along with the boron. This is an undesirable feature of this technique since fluorine has been known to exhibit adverse effects on the semiconductor device.
Cluster Implantation
A more effective way to increase the dose rate is to implant clusters of dopant atoms. That is, molecular ions of the form XnYm+, where n and m are integers and n is greater than one. Recently, there has been seminal work using boron cluster as a feed material for ion implantation. The implanted particle was a positive ion of the boron cluster molecule, B18H22, which contains 18 boron atoms, and is therefore a “cluster” of boron atoms. This technique not only increases the mass of the ion and hence the transportation energy, but for a given ion current, it substantially increases the implanted dose rate, since the boron cluster ion B18Hx+ has eighteen boron atoms. By significantly reducing the electrical current carried in the ion beam (by a factor of 18 in the case of boron cluster ions verses single boron atoms) not only are beam space-charge effects reduced, increasing beam transmission, but wafer charging effects are reduced as well. Since positive ion bombardment is known to reduce device yields by charging the wafer, particularly damaging sensitive gate isolation, such a reduction in electrical current through the use of cluster ion beams is very attractive for USJ device manufacturing. USJ manufacturing must accommodate increasingly thinner gate oxides and exceedingly low gate threshold voltages. Thus, there is a critical need to solve two distinct problems facing the semiconductor manufacturing industry today: wafer charging, and low productivity in low-energy ion implantation.
The principal means of forming shallower junctions in each of the last few generations has been through the reduction of annealing times (soak, spike, millisecond anneals) and overall thermal budget. While this approach produces shallower junctions with good activation, it makes the recovering of implant damage more difficult. In particular, the end-of-range (EOR) defects created by the widely used Ge pre-amorphization (PAI) implant typically survive low thermal budget treatments, resulting in higher junction leakage. Since the creation of end-of-range defects has proven to be a significant barrier to the fabrication of very low-leakage USJ devices, the fabrication of transistors with improved leakage characteristics is thus necessary to enable future generations of mobile devices. As illustrated below, ion implantation with clusters of boron and carbon allows for the elimination of all defects, and can achieve target USJ's for 45 nm, 32 nm, and smaller technology nodes.
Cluster ion implantation, or molecular implantation, has recently emerged as a production alternative for USJ formation. The use of cluster species dramatically increased wafer throughputs for the ultra-low energy implants required for USJ formation [1]. Cluster technology is now available for implants of B (B18Hx+), C (C16Hx+ or C7Hx+), As (As4+) and P (P4+). In addition, it is now becoming evident that the auto-amorphization feature of these implants allows for the elimination of the Ge PAI step, for example, as discussed in John Borland, Masayasu Tanjo, Dale Jacobson, and Takayuki Aoyama, Proceedings of the Eighth International Workshop on: Fabrication, Characterization, and Modeling of Ultra-Shallow Doping Profiles in Semiconductors, Jun. 5-8, 2005, Daytona Beach, Fla., USA, pp. 201-208, hereby incorporated by reference.
It has recently been reported in the literature John Borland et al., IEEE Extended Abstracts of the Sixth International Workshop on Junction Technology, May 15-16, 2006, Shanghai, China, pp. 4-9 and John Borland et al., IEEE Proceedings of the XVIth International Conference on Ion Implantation Technology, Jun. 11-16, 2006, Marseilles, France, hereby incorporated by reference, that B18Hx+-implanted junctions produce much lower photoluminescence (PL) and junction leakage signals than any of B, BF2, or Ge pre-amorphized samples when low-thermal budget SPE and laser anneals are used. By careful TEM analysis of implanted samples followed by anneal cycles of spike, SPE, laser, and flash techniques, it has been determined that implanting wafers with boron or carbon clusters of sufficient dose to amorphize the silicon results in clean annealed junctions with no observable EOR defects. While the theoretical basis for this effect is still emerging, it is clear that the implantation of a cluster of light atoms is fundamentally different from the implantation of a monomer ion.
In the case of the implantation of ions formed from B18H22, it has been established that simple substitution of this specie, with the appropriate accounting of the implanted dose (multiplying the measured dose by 18) and adjustment of the implantation energy (dividing the extracted ion energy by 20 for boron, and by 4.3 for BF2) for the boron or BF2 implant can not only match the implanted profiles, but largely eliminates channeling due to the auto-amorphization properties of implanting this cluster, for example, as disclosed by Y. Kawasaki, T. Kuroi, K Horita, Y. Ohno and M. Yoneda, Tom Horsky, Dale Jacobson, and Wade Krull, Nucl. Inst. Meth. Phys. Res. B 237 (2005), pp. 25-29, hereby incorporated by reference. A surprising side-benefit of this substitution is the observed absence of defects in the annealed junctions produced by this method.
One measure of performance of a semiconductor is the amount of leakage current across the junctions. Leakage current results from defects in the crystalline structure of the substrate caused by the implantation of the dopant. Although there have been efforts to reduce the defects and thus reduce leakage current in semiconductor junctions, leakage currents in available semiconductors are still at unacceptable levels. Thus, there is a need to enhance the performance of semiconductor junctions by further reducing leakage currents.